I always liked the mirrow inside   of the sideboard about 5 inches on each end or even with the bevel (flat part of top before drop off) on the sideboard top. You can always find a mirror r have one cut r yours shortened. To work with what you have, center the sideboard under the mirror and do your normal placement of what ever you are going to use on the top. A good center piece with take the wandering eye to the center and not the edges. 3 1/2 inches is a lot of over hang. Beat the bushes and get a mirror that fits.